Conventionally, a display device for a vehicle etc. is known. The display device has a liquid crystal display panel for displaying an image, and a light source for illuminating the liquid crystal display panel. The liquid crystal display panel may be called an LCD. It is also known that a gradient level of a pixel on the LCD and a source luminance of the light source are variable, and are adjusted in accordance with an adjust value for adjusting a brightness or luminance of a displayed image.
For example, a display device disclosed in the patent document 1 holds a gradient level of an LCD and varies a source luminance of a light source, when a user set value which acts as an adjust value is brighter than a predetermined value. Contrary, the device holds the source luminance and varies the gradient level of the LCD, when the user set value is darker than the predetermined value. The device enables an image luminance to be varied over a wide range in accordance with the user set value. In detail, the device enables to vary the image luminance to be varied by a variable control of the gradient level in a range where an adjustment of the image luminance is difficult by the source luminance alone. As a result, it is possible to realize a legible image display which better fits the user's preference.    Patent document 1: JP-H11-194736-A